An ignition coil is used for igniting an ignition plug of an internal combustion engine. The ignition coil includes a center core, a primary coil, and a secondary coil. The primary coil includes a primary winding that is wound around a primary spool. The secondary coil includes a secondary winding that is wound around a secondary spool. Current, which is supplied to the primary coil, is shut, so that high voltage is generated in the secondary coil. Both ends of the primary winding and the secondary winding are respectively connected with predetermined terminals by welding. The number of turns of the secondary winding is much greater than the number of turns of the primary winding. The diameter of the secondary winding is much smaller than the diameter of the primary winding. Accordingly, the secondary winding, which is significantly small in diameter, is difficult to be connected with a small-diameter secondary terminal arranged on the high-voltage side.
A conventional connecting method of a coil wire, which is disclosed in JP-A-2003-45735, includes three processes.
As shown in FIGS. 8A, 8B, 9, a coil wire 81, which has an electrically insulative sheath, is wound around a secondary spool 80, and a wire end of the coil wire 81 is wound around a coil terminal 82, to which air is supplied from air nozzles 87 in the first process.
Arc is generated between the coil terminal 82 and an electrode 86 of a torch 85, so that heat is generated in the coil terminal 82, and the electrically insulative sheath is flashed from the coil wire 81 of a winding portion 83 in the first heating step of the second process.
Arc is generated between the coil terminal 82 and the electrode 86 of the torch 85, so that the coil terminal 82 is welded, and the coil terminal 82 is connected with the coil wire 81 of the winding portion 83 in the second heating step of the third process.
The object of the conventional connecting method is to remove the electrically insulative sheath and to connect the coil wire 81 with the coil terminal 82 using a simple method, in which the coil terminal 82 is heated in twice. The electrically insulative sheath is removed from the coil wire 81 in the first heating step, i.e., second process. The coil terminal 82 is welded in the second heating step, i.e., third process after removing the electrically insulative sheath of the coil wire 81.
When, a secondary wire is wound to a secondary terminal for a significantly large number of turns such as 20000 turns, the secondary terminal becomes thin. In this case, the diameter of the secondary wire becomes significantly small, such as 50 μm. The conventional connecting method may be applied to the thin secondary terminal, to which the secondary wire of significantly small-diameter is wound for a significantly large number of turns. In this case, degree of electric field increases, and stress concentration arises due to acute shape of the connecting portion, in which the secondary terminal and the secondary wire are welded. Accordingly, an electric tree may be formed and cracking may be caused in an electrically insulative resin located around the connecting portion, as thermal cycles, in which the secondary coil assembly is heated and cooled, is repeated.
In the conventional connecting method, the tip end of the connecting portion is in a bulb-shape, however, increase of electric field and cracking of the electrically insulative resin are not described. Besides, the number of turns of the secondary winding, the diameter of the secondary winding, and the electrically insulative resin around the connecting portion are not described. In the conventional connecting method, the number of turns of the secondary winding may be small, and the diameter of the secondary winding may be large according to the description of the structure. In this structure, the secondary terminal may be thick, and increase of electric field in the tip end of the connecting portion may not be a serious problem. However, voltage generated in the secondary coil may be low, when the number of the winding of the secondary coil is low. Besides, the heating process is divided into two processes in the conventional connecting method. Accordingly, the connecting process takes long, and manufacturing cost may be increased in the conventional connecting process.